\documentclass{article}
\begin{document}
\section{questions}
\begin{itemize}
    \item What is Circuit?

    \item Inside the circuit,what are circuits composed of?
    
    \item What is node?What is element?
    
    \item If we classify digital circuits as two categories,what are these?Try to explain them in a simple way.
    
    \item There are three conditions when a circuit is combinational if it consists of interconnected circuit elements.List them.
\end{itemize}



\section{Answers}
\begin{itemize}
\item A circuit can be viewed as black box with  
one or more discrete-valued input terminals.  
one or more discrete-valued output terminals.  
a functional specification descripting the relationship between inputs and outputs.  
a timing specification descripting the relationship between input changing and output responding.

\item A circuit is composed of nodes and elements.    
  
\item A node is a wire whose voltage conveys discrete-value variable.  
A element is also a circuit with inputs,outputs and (functional and timing)specifications.

\item The two categories of digital circuits are combinational and sequential.  
A combinational circuit's output only depends on the current inputs values.  
A sequential circuit's output not only depends on the current inputs,but depends on the previous inputs.  
So a combinational circuit is memoryless,while the sequential circuit has memory.

\item Every circuit element is itself combinational.   
Every node of the circuit is either designated as an input to the circuit or connects to exactly one output terminal of a circuit element.    
The circuit contains no cyclic paths: every path through the circuit visits each circuit node at most once.  
\end{itemize}
\end{document}